1. Field of the Invention
The invention relates to a moisture-crosslinkable polyurethane adhesive which can be used in particular in the field of hygiene.
2. Description of Related Art
It is now commonplace to use fixing systems of the VELCRO® type to prepare disposable articles relating to the field of hygiene, in particular diapers for babies or incontinent adults and feminine hygiene articles. It is well known that the systems of the abovementioned type contain two components: a first component composed of a multitude of fine hooks and a second component composed of a multitude of fine loops. By simply pressing on the superimposed components, the hooks are made to enter the loops, which makes it possible to obtain a solid assembly. The bonding between the two components can be broken by the application of a sufficient force. The two components can be assembled and separated several times without a significant decrease in the strength of the assembly.
In the specific case of disposable diapers, these fixing systems are arranged on the rear and front parts forming the belt of the diaper when it is worn by the user (see U.S. Pat. No. 5,176,670).
The component containing the loops is fixed to the front part of the diaper, the loops being directed outwards. The component is generally composed of a thin material based on polyolefin fibers or on other synthetic fibers, in particular a knitted fabric (also known as textile mesh). The component is generally provided in the form of a strip of variable size, optionally decorated with patterns, which is adhesively bonded to the outer covering of the diaper. This covering can consist of a film which is impermeable to liquids, for example a polyethylene or polypropylene film, or of a non-woven which is impermeable to liquids and permeable to vapor.
The adhesive bonding of the component containing the loops to the impermeable material can be carried out by means of thermoplastic adhesives of the polyolefin or urethane type (see CA-A 2,122,942) or of moisture-crosslinkable polyurethane adhesives.
The adhesive bonding is carried out industrially at a high rate, that is to say at a speed which can exceed 150 meters per minute. The adhesive-bonding devices operate by coating the impermeable material with the liquid adhesive and laminating the component containing the loops. The assembly, thus formed, is subsequently subjected to calendaring and to winding off.
The amount of adhesive deposited must be carefully controlled: an amount which is too low does not make possible satisfactory adhesive bonding and an amount which is too high passes through the pores of the laminated component, with the risk of spreading over the loops, which is harmful to the fixing of the component containing the hooks.
It is economically advantageous to seek to limit the amount of adhesive while maintaining the performance of the adhesive bonding. To this end, the Applicant Company has sought to deposit the adhesive on the component containing the loops.
Under the abovementioned industrial conditions, the adhesives of the prior art have a tendency very quickly to no longer adhere to the component, which results in the adhesive wicking up on the dispensing device and the production line shutting down.
As regards adhesives based on moisture-crosslinkable polyurethanes, an additional constraint is imposed by the level of residual isocyanate monomer, which must be as low as possible for obvious toxicity reasons.